In a case where any component included in a biosample such as blood, urine or cerebrospinal fluid is detected, usually a series of analytical processes such as absorption, diffusion, reaction, detection, etc. of a sample fluid are required. In particular, in blood tests using blood including blood cells and plasma (hereinafter, sometimes referred to as “whole blood”) where glucose in plasma is to be detected and quantified as a substance to be detected, and so on, it is known that blood cells give various influences on measured results and tend to cause errors. For this reason, in blood tests using whole blood, it has been desired to separate blood cells and plasma. Various proposals have been made on techniques for separating blood cells and plasma from whole blood and detecting a substance to be detected in the separated plasma.
As the above-mentioned techniques, a testing instrument is known which has a mechanism of separating blood cells by centrifugation by utilizing a difference in specific gravity between blood cells and plasma. However, this testing instrument requires a centrifuge so that the device for detecting a substance to be detected becomes complicated and of a large scale and reagents used may become complicated so that there are cases where it is practically undesirable.
On the other hand, as the above-mentioned technique is known a test strip that separates blood cells from whole blood and detects a substance to be detected in plasma by using a coloring reagent that develops a color with the substance to be detected. The test strip is superior to the above-mentioned testing instrument in that it can detect a substance to be detected only by spotting it with whole blood.
As such test strips are known, for example, multi-layered type test strips having overlaid a blood cells separating layer having a porous structure composed of an asymmetric film, a glass filter, or polymer particles fixed with an organic polymer and a reagent layer comprising a coloring reagent that develops a color with a substance to be detected on an optically transparent substrate (for example, JP 49-53888 A, JP 55-90859 A, etc.), and test strips composed of polymer particles fixed with an organic adhesive or the like having suitable interstices between the beads as well as forming a layer comprising the above-mentioned coloring reagent, in which separation of blood cells, development of plasma, and detection of a substance to be detected are performed (for example, JP 57-160063 A, etc.).
However, in the above-mentioned multi-layered type test strip, whole blood tends to spread on the blood cells separating layer and the separation of blood cells and plasma is mainly separation in the vertical direction utilizing their own weights, so that a blood cells layer is formed on the top of the blood cells separating layer. Therefore, for example, when it is intended to quantify a substance to be detected by measurement of transmitted light, if the substance to be detected develops a color by the coloring reagent, there are cases where measuring light is cut off by the above-mentioned blood cells layer to make measurement of transmitted light impossible.
Also, there are cases where a substance to be detected is detected and measured while oxygen is being supplied thereto depending on the substance to be detected. In such cases, the above-mentioned type test strips tend to form a blood cells layer on the blood cells separating layer, resulting in poor permeability to oxygen; for supplying oxygen, it is necessary to provide an oxygen supply means such as an oxygen permeating membrane between a reagent layer and a substrate. To provide the oxygen supply means such as an oxygen permeating membrane to the above mentioned laminate type test strip is difficult and becomes a cause of increasing cost for fabricating the test strip.
Also, in the above-mentioned laminate type test strips, whole blood tends to spread on the blood cells separating layer and the separation occurs mainly in the vertical direction, and hence in order to sufficiently develop the substance to be detected in the reagent layer, it is necessary to spot a sufficient amount of blood to the blood cells separating layer, so that a larger amount of specimen is required for a single measurement.
Also, in the case of, among the above-mentioned multi-layered type test strips, those test strips in which the blood separating layer is formed by fixing polymer particles with an organic polymer, the organic polymer tends to absorb water and swell, the interstices between particles in the blood cells separating layer make too small to develop plasma, so that there may be the case where a sufficient amount of plasma is not developed in the reagent layer.
Also, in the case of the test strips disclosed in JP 57-160063 A, etc., suitable interstices between particles are formed in the layer so that plasma can be developed not only in the vertical direction but also in the horizontal direction due to a capillary action, which is advantageous in performing the above-mentioned measurement of transmitted light. However, because like the above-mentioned multi-layered type test strips, blood cells tend to spread on the reagent layer, there remain problems of the light shielding of the blood cells layer and the deterioration of oxygen permeability associated with formation of the blood cells layer in the measurement of transmitted light. Also, in these test strips, when plasma is developed in the layer, there are cases where dyes and the like generated by the reaction of it with a coloring reagent are further developed so that a deviation may occur in the concentration distribution of the substance to be detected.
Also, since the test strips are produced by fixing polymer particles with an adhesive or the like so that suitable interstices between particles can be formed, their production is complicated and difficult.